Comment by derefr
20 days ago
> Coming from a software background, this seems bizarre, as if C++ compilers rejected valid programs unless they stuck to easy constructs with obvious assembly implementations.
To my understanding, isn’t it more like there being a perfectly good IR instruction coding for a feature, but with no extant ISA codegen targets that recognize that instruction? I.e. you get stuck at the step where you’re lowering the code for a specific FPGA impl.
And, as with compilers, one could get around this by defining a new abstract codegen target implemented only in the form of a software simulator, and adding support for the feature to that. Though it would be mightily unsatisfying to ultimately be constrained to run your FPGA bitstream on a CPU :)
The non-synthesizable features of Verilog not only work in current simulators, they were expressly developed for that purpose. Verilog has those features to describe conditions that might exist in a semiconductor as manufactured, but aren't part of any design, so that they can be more accurately simulated. For example, a pin state can be unknown, or two pins can be connected with a delay line. These allow a real-life semiconductor to be characterized well enough to insert into a simulation of the electronics circuit as a whole.
It's more akin to directives than instructions. Debug instructions can also serve a similar purpose, although they actually run on the hardware, whereas compiler directives and non-synthesizable verilog instructions are never executed.